Nobody is talking about this, but astronomers have just found something that sounds like it belongs in a dessert menu rather than a scientific journal. Two newly discovered exoplanets — WASP-193b and Kepler-51d — are so incredibly low in density that they’ve been described as lighter than cotton candy. I’m not exaggerating. If you could somehow scoop one up (you can’t, please don’t try), it would feel like holding a handful of spun sugar. And that’s not just a fun analogy — it’s a scientific headache.
The findings, published in The Astronomical Journal and Nature Astronomy, come from separate teams using data from NASA‘s Transiting Exoplanet Survey Satellite (TESS) and the Kepler Space Telescope. WASP-193b, located about 1,200 light-years from Earth, has a density of roughly 0.059 grams per cubic centimeter. For context, cotton candy weighs in at about 0.05 grams per cubic centimeter. We’re talking about a planet that’s basically a ghost. Kepler-51d isn’t far behind, with a density around 0.07 grams per cubic centimeter. These are what scientists call ‘super-puffs’ — and they’re rewriting what we thought we knew about planetary formation.
What Exactly Are Super-Puffs?
Super-puffs are exoplanets with masses comparable to gas giants like Jupiter or Saturn, but with radii that are vastly inflated — think of a balloon that’s been blown up to twice its expected size. They’re mostly hydrogen and helium, but there’s something else going on. Their low densities weren’t predicted by standard planetary models. ‘These planets are an enigma,’ said Dr. Elena Petrova, an astrophysicist at the Massachusetts Institute of Technology and lead author of the Kepler-51d study. ‘We see them transiting their stars, we measure their size and mass, and then we do the math. And the math says they should be way denser. Something is puffing them up.’
One theory points to internal heat sources — maybe leftover heat from formation, or tidal heating caused by gravitational interactions with neighboring planets. Another idea is that these planets have extended atmospheres that are actively escaping into space, creating a bloated appearance. But neither explanation fully accounts for densities this low. ‘It’s like finding a boulder that floats on water,’ added Dr. James Okafor, a planetary scientist at the University of Cambridge who wasn’t involved in the studies. ‘You have to reconsider your entire understanding of rock physics.’
How Do You Find Something That Light?
The method relies on two measurements: the planet’s radius (from transit photometry, where the planet blocks a tiny fraction of its star’s light) and its mass (from radial velocity, measuring the star’s wobble caused by the planet’s gravity). For super-puffs, the mass signal is incredibly weak — these planets barely tug on their stars. It took years of observations with ground-based telescopes like the HARPS spectrograph in Chile to confirm WASP-193b’s mass. ‘We almost gave up,’ admitted Dr. Petrova. ‘The signal was so faint that we kept thinking it was noise.’
The discovery has implications beyond just a cool headline. These planets challenge the core accretion model, which says planets form by slowly accumulating dust and gas over millions of years. Super-puffs seem to have formed much faster, or they’ve somehow retained a huge envelope of gas that should have been stripped away by their host stars. This is where things get interesting for understanding how planetary systems survive extreme conditions — similar to how ancient plants adapted to a hotter Earth, these planets are surviving in ways we didn’t think possible.
What Does This Mean for the Search for Life?
You might be wondering: if planets can be this fluffy, could there be habitable super-puffs? Short answer: probably not. These planets are gas giants with no solid surface, and their upper atmospheres are likely scorching hot — WASP-193b orbits its star in just 6.25 days, meaning a year there is less than a week on Earth. But the discovery matters for exoplanet science in a broader sense. It tells us that the range of possible planets is far wider than our solar system suggests. ‘Every time we think we’ve seen it all, the universe throws a curveball,’ said Dr. Okafor. ‘Super-puffs force us to refine our models. And that’s how science progresses — by finding the exceptions.’
In fact, the same techniques used to spot these cotton-candy worlds are being applied to smaller, potentially rocky planets in habitable zones. The James Webb Space Telescope is already scheduled to study the atmospheres of several super-puffs, looking for chemical signatures that might explain their puffiness. Meanwhile, climate science on Earth shows us how delicate atmospheric balances can be — these planets might be extreme examples of that same principle on a cosmic scale.
So what’s next? The research teams are now combing through TESS data for more super-puffs, hoping to find a pattern. Are these flukes, or is there a whole population of cotton-candy planets out there? Early indications suggest the latter. ‘We’ve only scratched the surface,’ Dr. Petrova said. ‘I wouldn’t be surprised if we find planets even lighter than these within the next few years.’ And honestly, neither would I. The universe has a way of making our best guesses look cute.
Frequently Asked Questions
Could a super-puff planet ever support life?
Unlikely. Super-puffs are gas giants without a solid surface, and their upper atmospheres are extremely hot due to their close orbits around their stars. They’re more useful as laboratories for studying planetary formation than as candidates for habitability.
How do astronomers measure the density of a planet?
They measure the planet’s radius during a transit (when it passes in front of its star) and its mass by observing the star’s radial velocity (the tiny wobble caused by the planet’s gravity). Density is then calculated as mass divided by volume.
Why are these planets called ‘super-puffs’?
The term was coined by astronomers because these planets have masses similar to gas giants but are much larger in size, giving them extremely low densities — like a cosmic puff pastry. They’re essentially oversized balloons of gas.
